1. Field of the Invention
This invention relates to a voltage regulator for an alternating current generator of an automobile, etc., which has two distinct output voltage levels a lower voltage output for charging a battery, etc., and a higher voltage output for an electrical load operating at a voltage higher than that of the battery.
2. Description of the Prior Art
Automobiles are generally equipped with an alternating current generator which is used to charge a battery and supply power to electrical equipment thereof. Usually, such generators have only one output voltage level, at which the battery is charged and the electrical equipment is operated. In some cases, however, automobiles have electrical loads which are operated at a voltage level higher than that of the battery and other electrical equipment. For example, automobiles used in extremely cold regions may have heaters for rapidly melting ice that often forms on the wind shield, etc., thereof, such heaters requiring a voltage higher than the battery voltage. In such cases, the output of the alternating current generator must be regulated to two distinct voltage levels: to the lower level when operating in the normal mode to charge the battery, and to the higher level when temporarily operating in the high voltage mode to supply power to the high voltage electrical load such as an ice melting heater mentioned above.
FIG. 1 shows a circuit diagram of a conventional voltage regulator and switching circuit for an AC generator of an automobile having two distinct output voltage levels. An AC generator driven by the engine of the automobile includes a stationary armature winding 101 and a field winding 102 mounted on a rotor. A full-wave rectifier circuit 2 rectifies the AC voltages induced in the armature winding 101 and outputs a DC voltage E1 across the positive and negative output terminals 201 and 202 thereof, the negative output terminal 202 of the rectifier being grounded The voltage regulator circuit 3, which regulates the output voltage of the generator to two distinct predetermined levels by controlling the field current, comprises a voltage divider circuit consisting of a series connection of resistors R1 and R2, a switch 301, and a resistor R3 connected and disconnected from the resistors R1 and R2 by the switch 301. The series connection of resistors R1 and R2 constitutes the voltage divider for detecting the battery voltage E2, while the series connection of resistors R1 through R3 constitutes the voltage divider for detecting the higher operating output voltage supplied to the load 5, the junction between the resistors R1 and R2 constituting the detecting point, as explained below. The voltage regulator 3 further comprises a Zener diode 302, a transistor 303, which becomes conductive when the Zener diode 302 suffers a reverse-voltage breakdown (Zener breakdown), and a power transistor 304 controlling the current through the field winding 102. The power transistor 304, which has a base coupled to the positive terminal of field winding 102 through a resistor R4, is turned off when the transistor 303 is turned on. A diode 305 is coupled across the field winding 102 to absorb the surge that is generated when the current through the field winding 102 is turned off. Output voltages of the generator at lower and higher levels are supplied to battery 4 and high voltage load 5 (such as a heater for melting ice on the windshield, etc., operated at a voltage higher than the battery voltage E2), respectively. The switching of the output between the two voltage levels supplied to the battery 4 and the load 5, respectively, is effected by the switching circuit 7 and the switch 301 for selecting the voltage divider, the circuit 7 including an output change-over switch 71 and an electromagnetic switch 72 coupled in series with the key switch 6.
The operation of the circuit of FIG. 1 is as follows. In the normal operation mode, the change-over switch 71 and the voltage divider switch 301 are coupled to the battery 4, as shown in the figure, and when the key switch 6 is closed to start the engine of the automobile, the electromagnetic switch 72 is automatically turned on. As a result, a current is supplied from the battery 4 to the field winding 102 of the generator 1. When the engine is started, the engine drives and rotates the field winding 102 with respect to the armature winding 101. Thus, AC voltages are induced in the armature winding 101 of the generator 1, and rectifier 2 outputs a DC voltage E1 across the output terminals 201 and 202 thereof As can be easily seen from the figure, the output voltage E1, the battery voltage E2, and the voltage E3 at the positive terminal of the voltage divider consisting of resistors R1 and R2 are substantially equal in this operation. Thus, when the output voltage E1 exceeds the lower predetermined level to raise the voltage E0 at the junction between the resistors R1 and R2 above the Zener breakdown voltage, the Zener diode 302 becomes conductive in the reverse direction to turn on the transistor 303, thereby turning off the power transistor 304. Thus, supply of current from the battery 4 to the field winding 102 is stopped. When, as a result, the output voltage E1 becomes lower than the predetermined level, the Zener diode 302 regains non-conductivity in the reverse direction, and the transistor 303 is turned off, thereby turning on the power transistor 304. By repeating these operations, the regulator 3 regulates the output voltage E1 to the lower predetermined level in the normal mode, which level is equal to the voltage E2 across the terminals of the battery 4.
When the operation is to be switched from the normal to the high voltage mode, the electromagnetic switch 72 is first opened to attenuate the current through the field winding 102, so that damage to the change-over switch 71, etc., caused by the inductance of the field winding 102 at the switching operation may be avoided. After a predetermined period of time at the end of which the field current has been sufficiently attenuated, the change-over switch 71 is coupled to the high voltage load 5, while the voltage divider selecting switch 301 is coupled to the resistor R3; thereafter, the electromagnetic switch 72 is made again (The sequential control of these switching operations are effected automatically by a switching operation control circuit (not shown) comprised in the circuit of FIG. 1.). Thus, the field current is supplied from the battery 4 to the field winding 102 being driven by the engine, and the resulting DC output E1 across the rectifier 2 is supplied to the high voltage load 5. In this condition, the voltage divider consists of three resistors R1 through R3, the detecting point being at the junction between the resistor R1 and R2. When the voltage E0 at the junction between the resistors R1 and R2 exceeds the Zener breakdown voltage, the Zener diode 302 becomes conductive in the reverse direction to turn on the transistor 303, thereby turning off power transistor 304. Thus, the output voltage E1 is regulated to the higher predetermined voltage which is required for the load 5. Also during this high voltage mode operation, the battery 4 keeps on supplying current to the field winding 102 without being charged by the generator. Thus, to prevent the over-discharge of the battery 4, the duration of the high voltage mode operation is limited to a short interval of time not exceeding about 5 minutes. When the voltage E2 across the battery 4 becomes lower than a predetermined level, the circuit stops the high voltage mode operation and returns to the normal mode. The sequential control of these switching operations are also effected by the switching operation control circuit associated with the switches 71, 72 and 301.
The conventional voltage regulator and switching circuit shown in FIG. 1 has the following disadvantages: First, the provision of the voltage divider selecting switch 301 increases the number of parts to be assembled, and hence production costs thereof; second, the voltage drop across the voltage divider selecting switch 301 increases with time due to deterioration thereof, thereby raising the lower and the higher regulated voltage in the normal and the high voltage mode operation, respectively; third, if the switch 301 breaks and the voltage drop thereacross becomes infinitely great, the voltage regulator ceases to function, and the field current is continuously supplied from the battery 4 irrespective of the output voltage E1, which, in consequence, is raised to an abnormally high level, thereby over-charging and damaging the battery 4 or injuring the high voltage load 5.
Japanese Laid-open Utility Model Application Nos. 55-87140 and 62-88500 also disclose voltage regulators for an alternating current generator with two output voltage levels. The first named application teaches a voltage regulator for an AC generator with two DC output voltages, while the second teaches a regulator for an AC generator with a low DC and a high AC output voltage. In both cases, however, the switching between the voltage detecting circuits for the two output voltages is effected by means of switches, as in the case of the conventional circuit shown in FIG. 1.